Integral color anodizing of aluminum

ABSTRACT

A METHOD OF INTEGRAL COLOR ANODIZING OF ALUMINUM COMPRISING SUBJECTING THE ALUMINUM AS THE ANODE TO ELECTROLYSIS IN AN AQUEOUS ELECTROLYTE HAVING A PH BETWEEN 0.5 AND 2.0 AND CONTAINING COMPOUNDS CAPABLE OF COMPLEXING WITHTHE ALUMINUM WHICH DISSOLVES IN SOLUTION DURING ANODIZNG. THE ELECTROLYTE IS PREFERABLY ACIDIFIED WITH SMALL AMOUNTS OF SULFURIC ACID, OXALIC ACID, AND THE LIKE.

United States Patent 3 714 000 HJ'I'EGRAL COLOR ANOEIZING F ALUMINUM Geoffrey A. Dorsey, Jr., Danville, Calif., assignor to Iaiser Aluminum & Qhemical Corporation, Oakland,

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No Drawing. Fiied Nov. 29, 1971, er. No. 203,024 Int. Cl. C231) 9/02 US. Cl. 204-58 3 Claims ABSTRACT OF THE DISCLOSURE A method of integral color anodizing of aluminum comprising subjecting the aluminum as the anode to electrolysis in an aqueous electrolyte having a pH between 0.5 and 2.0 and containing compounds capable of complexing with the aluminum which dissolves in solution during anodizing. The electrolyte is preferably acidified with small amounts of sulfuric acid, oxalic acid, and the like.

BACKGROUND OF THE INVENTION Heretofore many methods have been employed to place a protective oxide coating on aluminum surfaces, one of the more frequently used is the process of anodizing wherein the aluminum workpiece is made anodic in an electrolytic bath. The aluminum surface thus protected is commonly termed anodized. Most often the baths are acidic. As used herein, the term aluminum includes high purity aluminum, various commercial grades of aluminum and aluminum base alloys.

To produce an abrasion resistant coating, it has been necessary in the past to anodize in sulfuric acid baths at low temperatures such as 0 to 30 R, which entails considerable expense for refrigeration.

Under many circumstances, such as in architectural applications where aesthetic considerations are quite important, it is desirable to have a colored surface on the aluminum. Several processes have been developed to produce color oxide coatings. However, only one has met with real commercial success, that being the integral color anodizing process basically described in Re. 25,566, assigned to the present assignee. Methods such as coloring a previously anodized aluminum surface with organic dyes and introducing metallic salts or oxides into a previously prepared porous oxide coating suffer from the inherent disadvantage of requiring additional processing steps after anodizing which increase the cost of the process. The dyeing has the additional disadvantage of producing a color which fades when exposed to ultraviolet light, and also a color which is difiicult to reproduce from batch to batch.

The color anodizing process described in Re. 25,566 involves the anodizing of aluminum in an aqueous electrolyte containing sulfosalicylic acid and sulfuric acid. Other sulfonic acids such as 4 sulfophthalic acid, 5 sulfoisophthalic acid, sulforesorcinol, lignosulfonic acid and 1,8 dihydroxynaphthalene 3,6 disulfonic acid may be utilized in place of sulfosalicylic acid. The color anodizing process produces not only a wide range of colored oxide coatings which are light fast, but also an abrasion resistant oxide coating which does not require electrolysis at low temperatures. In general, the colors so produced are uniform, reproducible, and have an aesthetic appeal necessary for architectural applications. Although the integral color anodizing process described above has met with considerable commercial success, little information is known about the mechanism of the color formation during anodizing. The prior art has been limited to finding that an aqueous solution containing a particular aromatic sulfonated com- 3,714,000 Patented Jan. 30, 1973 DESCRIPTION OF THE INVENTION The present invention is directed to a novel electrolytic bath and the process of color anodizing aluminum surfaces in such a bath. More particularly, it is directed to the color anodizing of aluminum products in an acidified aqueous electrolyte containing at least one compound selected from a group consisting of 2,6 dihydroxybenzoic acid; 2,4 dihydroxybenzoic acid; 2,3 dihydroxybenzoic acid; 3 hydroxy 1,2 benzenedicarboxylic acid; 4 hydroxy 1,3 benzenedicarboxylic acid; 2 hydroxy 1,3 benzenedicarboxylic acid; 2 hydroxy 1,4 benzenedicarboxylic acid; 1,2,4 benzene tricarboxylic acid; 1,2,3 benzene tricarboxylic acid; and 4 hydroxy 1,2 benzenedicarboxylic acid.

The above-named organic compounds can be substituted with one or more constitutents selected from the group consisting of OH, COOH, -SO H, SO' H, R(OI-I) and -R wherein R is an alkyl or group con taining 14 carbon atoms and x is an integer from 1-4. Preferably, the electrolyte is acidified with small amounts of sulfuric acid, oxalic acid, malonic or maleic acid.

In the process of this invention, an aluminum article is immersed as the anode in an aqueous electrolyte preferably containing small amounts of sulfuric acid and one or more of the various organic compounds mentioned above and subjected to anodizing with a direct current. A plurality of electrical anodizing programs can be employed. However, the easiest to control, and most etficient, is the two-stage anodizing program which comprises first subjecting the aluminum article to a substantially constant current density between about 10 and 50 amps/ft. until a peak voltage between about 20 and volts is reached and subsequently maintaining the voltage at substantially this peak level until the desired color and oxide thickness is obtained. It is recognized that other electrical anodizing programs, such as maintaining the current density between about 10 and 50 amps/ft. at a constant current density provides substantially equivalent results. Care must be exercised during the initial or run-in period when first applying the voltage so as to maintain a uniform and regular growth rate for the oxide. Otherwise, irregular non-uniform nodular films may form. Non-nodular or uniform films are produced if the initial current is low; whereas high current densities produce nodular coating. Nodular films are a characteristic of the dielectric breakdown of the oxide coating. The bath temperature can range from 0 to 100 F., but is preferred to maintain the temperature between 60 and 95 F. The organic compo nent in the bath can range from about 0.1 to grams/ liter and the sulfuric acid or its equivalent can range up to 30 grams/liter, preferably from 1 to 10 grams/liter.

The present invention is based on the discovery and appreciation of the criteria necessary for an acidic electrolyte to form integrally colored anodic oxide coatings on aluminum. It has been found that the organic component in the electrolyte must be capable of complexing with aluminum ions during anodizing to form an anionic chelate. The electrolyte itself must have a pH between 0.5 and 2.0, and the organic components must have a solubility of greater than 0.1 gram/ liter.

It is believed that the organic component of the electrolyte complexes with the aluminum which is dissolved during anodizing. However, due to the strong anode field (voltage divided by the barrier layer thickness) at the interface of the workpiece and electrolyte, the chelate tends toward anodic instability, releasing Al+ During this period the oxide surface is losing hydrogen ions due to the field effect leaving A10-sites and thus giving the 3 surface a temporary net negative charge. The aluminum ions which are released from the chelate can then react at the aluminate site to form an oxide structure which is believed to have aluminum in both anionic and cationic form. Due to the presence of OH- in solution at the oxide-electrolyte interface, the aluminum ions also have a tendency to form an alumina hydrate which is somewhat different in structure from the usual boehmitic structure, herein termed pseudoboehmite. Pseudoboehmite, which is clear and colorless, deposits on the outer surface of the oxide. Below about 40 volts, depending on barrier layer thickness, the outside layer has a lesser tendency three electrolytes. The bath compositions, anodizing to lose hydrogen ions and thus most of the aluminum. ions parameters and results are given in Table I.

TABLE I Composition Bath Current Peak tcmp., density, voltage, Example number Organic component Acid C amps/rt. volts Color 1 Saturated 3,5-dihydroxy bcnzoic acid g.p.l. 11 50 Varied Varied Colorless. 2. Saturated 2,4 dihydroxy benzoic acid 80 10 90 Gray. 3". 1 g.p.l. 2,6 dihydroxy benzoic acid 0.1 g.p.l. H2804.-. 25 24 100 Black.

react with the OH to form the pseudoboehmite. As the anode field increases, the pseudoboehmite formation decreases in favor of the reaction of aluminum ions with the Al0-structure in the outside layer. This reaction is more dominant above 40 volts, again depending upon barrier layer thickness.

The oxide structure, which is believed to have both initially anionic and cationic aluminum, is usually the major factor in the generation of integral color in the anodic oxide layer. During the initial phases of anodizing, the voltage is low and the barrier layer thickness is small resulting in a low anodic field, and thus little generation of color, As the anodizing proceeds, the anode field increases enhancing the conditions for forming the integrally colored oxide, and the oxide color darkens. Because the anodic oxide builds up from the metal substrate, the first formed oxide at the oxide-air interface is relatively clear and slightly colored; whereas the later formed oxide closer to the oxide-metal substrate interface is considerably darker and more opaque.

Alloying constituents in the base metal are known to remain in the anodized oxide layer, and these occluded constituents also tend to darken the oxide coatings. The alloying constituents characteristically remain in the oxide adjacent to the metal substrate after reaching equilibrium with the solvation effects of the electrolytes with the outer portions of the oxide being substantially devoid of alloying constituents.

The net effect of the two above-described phenomena is to form an oxide layer, the outmost portion of which is relatively clear and slightly colored, and an innerportion of which is opaque and has a high color density. Although the color generation at the lower voltages may be relatively low, darker coatings can be obtained by increasing the thickness of the oxide coatingi.e. increasing the time of anodizing and total current quantity.

As is evident from the above discussion, the complexing ability of the organic component in the electrolyte is a necessary criterion for integral color formation during anodizing. It has been found that to obtain an electrolyte having at least minimal complexing ability for color anodizing, the organic component must have at least two complexing ligand constituents, preferably three, adjacent to one another on the benzene ring. Additional constituents on the benzene ring, such as -COOH, -OH, SO H and SO H tend to activate the ligand constituents to form more stable chelates. The SO H and -SO H groups substantially increase the solubility of the compounds useful herein, thus forming a more serviceable electrolyte, particularly that electrolyte containing slightly soluble organic components. A practical screening test to determine whether the compound has sufficient complexing ability to form integrally colored coatings is The electrolyte containing the 3,5 isomer produced clear, colorless anodic oxide coatings and no amount of variation in current density, voltage, bath temperature and the like would produce an integrally colored coating. From the fact that the constituents on the benzene ring of the 3.5 isomer are meta to one another, this compound has little or no tendency to complex with aluminum. This is further verified by the mixing of the electrolyte with a solution of ferric sulfate, which results in no substantial color change of the ferric sulfate solution. The electrolyte containing the 2,4 isomer produced a grey anodic oxide coating, This electrolyte, when mixed with a ferric chloride solution, produced a substantial color change. The 2,6 isomer has an extremely strong tendency to complex with aluminum due to the two hydroxy groups positioned ortho to the carboxyl group on the benzene ring. As indicated on the table, only small concentrations of the 2,6 dihydroxybenzoic acid and sulfuric acid are needed for color generation. This latter isomer is particularly unique in that extremely dark brown and black coatings can be obtained with an oxide thickness of about 0.1 to about 0.3 mil. Most, if not all, of the prior art electrolytes employing organic sulfonic acids require an oxide thickness of about '1 mil when anodizing to the darker browns and blacks. Due to the strong chelating activity of the 2,6 isomer, it is preferred to maintain the concentration of the isomer between 0.1 and 10 grams/liter and the concentration of the sulfuric acid between 0.1 and 3 grams/liter.

Although small amounts of an acidifying agent such as sulfuric acid are usually required in the electrolyte of the present invention to develop intense colors, an aqueous solution of some of the organic compounds of the present invention is adequate for integral color anodizing without the addition of sulfuric acid. These compounds, namely 4 hydroxy isophthalic acid and 2,6 dihydroxybenzoic acid, have a high complexing ability with aluminum, are sufiiciently soluble in the electrolyte and provide a solution with a pH between 0.5 and 2.0. A full range of colors, i.e. light tan or gold through various shades of brown to black, can be obtained by merely changing anodizing parameters such as current density, voltage, total current quantity and the like.

Although sulfuric acid is the usual and most convenient acidification agent, the electrolytes of the present invention can be acidified with other electrolytes such as oxalic acid, malonic acid and maleic acid. Acids such as nitric acid, hydrochloric acid and hydrofluoric acid are well known to be detrimental to the anodizing process as well as the anodic coating formed, and their use is not contemplated by the present invention. Acidifying agents should not exceed 50 grams/ liter because at this high concentration the electrolyte acts as if only the acidifying agent is present in the electrolyte, and only colorless coatings are obtained under normal anodizing conditions.

Thus the electrolytes of the present invention contain from 0.1 to 150 grams/liter of an organic component of the group described above and the electrolyte is acidified controlled by passing the electrolyte through a cation exchange resin.

After anodizing, the articles of the present invention are usually sealed in the normal fashion such as in boiling water or a dilute a ueous solution of nickel acetate with up to 50 grams/liter of an acidifying agent selected 5 q and sodium ignosulfonate.

from the group consisting of sulfuric acid, oxalic acid, The inte ran color d f th t malonic acid and maleic acid to control the pH of the y e c a mgs 0 e plesen mven' electrolyte between and tion are light fast and have excellent abrasion and cor- The examples given in Table II are typical examples reslslfanwillustrating the particular embodiments of the present in- It Dbl/1011s thflt Various modlficatlons can made vention and are used to illustrat th th t li it in the above-described invention without departing from the present invention. the spirit thereof and the scope of the appended claims.

TABLE II Bath composition Bath Current;

temp., density, Peak Example number Original component Ac p VOlt C0101 1 1.5 g.p.l. 2,6 dihydroxybenzoio acid 0.1 g.p.l. H2504 24 65 Black. 2-. Saturated 2 hydroxy-1,3 benzene dicarboxylic aci g.p 112304 5 5 Brown- 3 Saturated 4 hydroxy-1,3 benzene dicarboxylic acid 30 g.p.l. HzSO 25 35 Amber. 4-.-- Saturated 3 hydroxy-1,4 benzene dicarboxylic acid 5 g.p.l. H2804 25 22 Brown. 5.- Saturated 3 hydroxy-1,2 benzene dicarboxylic acid 25 $41.1. H23 4 2 0 50 Ten. 6-. 50 g.p.l. 4 hydroxy-1,2 benzene dicarboxylic acid- 5 g.p 1 H2804 15 25 65 Brown. 7-. Saturated 2 hydroxy-1,3,5 benzene tricarboxylic acid 24 Dark grey.

... Saturated 1,2,4 benzene tricarboxylic acid 50 g.p.l. H1804 25 50 30 Dark brown.

The alloy employed is a 6063 aluminum alloy (Aluminum Association designation) modified with 0.15% manganese and 0.35% copper. After the normal pretreatments such as degreasing in an inhibited alkaline cleaner, caustic etching, and desmutting in nitric acid, the aluminum article is anodized in the indicated electrolyte. The anodizing program is a constant current densityconstant voltage program. In the operation the constant voltage stage is continued until the indicated color is obtained.

With the electrolytes of the present invention, the colors obtained can be varied, usually between light brown or light gold through the various shades of brown or grey to black by changing suitable anodizing parameters. The parameters which affect color include the bath composition, bath temperature, current density, voltage, total current quantity and the like. Normally, the bath composition is maintained relatively constant and only the current density, voltage, and possibly bath temperature, are varied to obtain the desired oxide thickness and color. In some instances it is desirable to increase the temperature of the bath, for example up to 90 F., to facilitate the formation of the lighter colors such as the light gold or light brown.

As recognized by those skilled in the art, the aluminum concentration in the electrolyte increases as the anodizing proceeds. An aluminum concentration above 13 grams/liter detrimentally affects the process. Moreover, the aluminum concentration variation should be kept within a narrow range because large variations change the color obtained with a particular anodizing program making color match from batch to batch diflicult. On a commercial basis the aluminum concentration is usually What is claimed is:

1. A process for the integral color anodizing of aluminum articles comprising subjecting the article as the anode to electrolysis with a direct current in an aqueous electrolyte having a pH of 0.5-2.0 and consisting essentially of up to 5 grams/liter of at least one compound selected from the group consisting of sulfuric acid, oxalic acid, malonic acid and maleic acid, from 0.1 to grams/ liter of an aromatic compound selected from the group consisting of 2,6 di-hydroxybenzoic acid; 2,4 dihydroxybenzoic acid; 2,3 dihydroxybenzoic acid; 3 hydroxy 1,2 benzenedicarboxylic acid; 4 hydroxy 1,3 benzenedicarboxyli'c acid; 2 hydroxy 1,3 benzenedicarboxylic acid; 2 hydroxy 1,4 benzenedicarboxylic acid; 1,2,4 benzene tricarboxylic acid; 1,2,3 benzene tricarboxylic acid; 4 hydroxy 1,2 benzenedicarboxylic acid, and the balance Water.

2. The process of claim 1 wherein the aromatic compound has on the benzene ring at least one constituent selected from the group consisting of OH, COOH, SO H, -SO H, R and R(OH) where R is an alkyl or alkenyl group containing 1 to 4 carbon atoms and x is an integer from 1 to 4.

3. The process of claim 1 wherein the aromatic compound is 2,6 dihydroxybenzoic acid in amounts from 0.1 to 10 grams/liter.

References Cited FOREIGN PATENTS 447,421 5/1936 Great Britain 20458 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 

